The present invention relates to glass fiber-filled polyester resins and, more particularly, to the use in polyester resins of glass fibers with low adhesion for the resin to reduce the coefficient of linear thermal expansion of the polyester resin.
It is known to make exterior automotive parts, for example side cladding and rocker panels, out of moulded, glass fiber-reinforced thermoplastic polymer. Thermoplastic polyesters such as poly(butylene terephthalate)(PBT) or poly(ethylene terephthalate)(PET), for example, are stiff enough to be used as the thermoplastic polymer in such applications but are not sufficiently tough at low temperatures. Addition of thermoplastic elastomeric polyesters, for example a polyetherester (PEE), increases toughness. Poly(etherimide)esters are known to have properties very similar to polyetheresters and may be used in place of some or all of the polyetherester toughener. The polymer blend itself has a coefficient of linear thermal expansion (CLTE) which is too high for the demanding requirements of exterior automotive parts and glass fiber is incorporated in the resin to reduce CLTE.
One such fiber-reinforced polyester resin is a thermoplastic polyester composition of which the basic components are a crystalline segmented block copolymer with elastomeric soft segments and high melting crystalline polyester hard segments (a thermoplastic elastomeric polyester) and a thermoplastic, high modulus, high melting crystalline polyester. The thermoplastic elastomeric polyester used is a polyetherester containing poly (butylene terephthalate) (PBT) hard segments and polytetramethyleneetherglycol (PTMEG) soft segments. The high modulus, high melting cyrstalline polyester is PBT.
It is well known that the fiber in polyester blends must act as a reinforcement by the use of an appropriate coupling agent on the glass fiber to strongly adhere the fiber to the polyester. However, adequate reinforcement, while it reduces CLTE, results in a significant reduction in toughness as measured by elongation at break, Rheometrics impact strength or the falling dart impact test. The notched Izod impact test is considered to be not of great sensitivity in studies on fiber-filled resins, particularly because most of the fibers are aligned perpendicular to the impact pendulum.
Also commercially important is the surface quality of polyester compositions used in exterior automotive applications and the ability to paint exterior parts efficiently. The surface appearance of the exterior parts must be maintained over time. "Paintability" requires heat sag resistance whereby less support of the parts is needed during painting. Surface quality requires that fiberrelated surface defects should be avoided; examples of such defects are weld lines, waviness and warpage. Fiber-related surface defects are exacerbated by increasing glass fiber level.
The present invention aims to provide fiber filled resins which have a reduced CLTE as compared with the unfilled resin but do not have the reduction in toughness associated with previous fiber-filled resins.